Pipe laying - bridge crossing
 
Bridge crossing with a main consisting of socket components involves determining:
  • the support system,
  • the accommodation of both bridge and main thermal expansion,
  • the anchoring of components subjected to hydraulic thrust.
There are two principal installation systems which can be used depending on the type of project:
  • pipeline secured to the structure,
  • pipeline independent of the structure.
The systems shown below correspond to the usual types of bridge crossings; they are given as examples and do not represent the variety of situations that may be encountered.
 
Every bridge is a particular case and must be studied specifically. The initial concern is to ensure that the structure can support the weight of the mains and that the provision of anchorages is possible.
 
PIPELINES SECURED TO THE STRUCTURE
Typical support / cross section
Traditional masonry structure
Typical support / Cross section
Open end structure
 
Supports
  • one per pipe.
  • every support behind a socket.
  • supporting saddle ( = 120° is advisable).
  • securing clamp.
  • rubber protection between the support and the pipe.
Thermal expansion
Relative expansion: every clamp must be adequately secured to constitute a fixed point with the bridge. Between each of these supports, which are solid with the pipe and bridge, the elastic joints act as expansion compensators, absorbing the expansion of a length of pipe.

Overall expansion (L): overall expansion at the bridge ends is accommodated by either a simple elastic joint (in the case of a traditional masonry structure) or by a piece of equipment that acts as an expansion compensator (in the case of an open end structure).
 
Anchorage
Every component subjected to hydraulic thrust (bends, tees, valves... ) must be held by an anchoring system. The supports must be of sufficient dimensions to secure the correct alignment of the pipes and to withstand hydraulic thrust. Provision of an adequate safety margin on the support dimensions is recomended, to compensate for hydraulic forces due to any misalignment of the pipes.
 
PIPELINES INDEPENDANT OF THE STRUCTURE
Cross section
Pipe with anchored joint
 
Supports
Each support is solid with the pipeline and independent of the bridge movements. Several techniques may be used, depending on the magnitude of the expansions; rolling on rails or rollers etc.

The support sliding forces must be compatible with the pipeline anchoring system:
  • one support per pipe,
  • a support behind each socket,
  • supporting saddle,
  • securing clamp,
  • rubber protection between the pipe and the support.
Thermal expansion
The pipeline expands and contracts independently of the bridge. The joints are self-anchored: this facilitates assembly and plays a part in distributing the overall pipeline expansion. This expansion L is transferred to a free end of the main by an expansion absorber.
 
Anchorage
Every component subjected to hydraulic thrust (bends, tees, valves...) must be stabilized by an anchorage system.

Sliding supports must be of adequate size to maintain the pipeline correctly aligned and to withstand the effects of hydraulic thrust. Provision of an adequate safety margin on the support dimensions is recommended, to compensate for hydraulic forces due to any misalignment of the pipes.
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